Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

This can also be a problem with the sessionstore file(s) that take too long to update. You can check browser.sessionstore and try to increase the value of this pref (default is 15 sec.).

• browser.sessionstore.interval

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

Thanks you for your response. I followed the above directions. That did NOT help, right / left arrows are still jerky. All other keys seem fine, just the right and left arrow keys. The following small html/javascript shows the problem:

<font size=5 color=red>
  Firefox: Holding down left/right arrow near the bottom of textarea, is jerky<br>
  Chrome, IE: ok </font><P>

<textarea id=ta1  rows="500" cols="80"> </textarea>
  
<script>

   //----- Build textarea data (to edit)
   var msg = "";
   for(var i=0; i<500; i++) {
      msg = msg + "Line " + i + 
            ": Some test data to make this line of text fairly long\n"; 
   }
   document.getElementById("ta1").value = msg;

</script>